In mobile radio communications, a variety of different type radio channels may be used to convey different types of information. For example, radio channels may be defined as control signaling channels or traffic channels, or they may be characterized as dedicated or common/shared channels. In third generation (3G), wideband code division multiple access (WCDMA) cellular communications systems, the physical radio channels are categorized in many ways: primary common control physical channel, secondary common control physical channel, physical random access channel, dedicated physical data channel, dedicated physical control channel, physical downlink shared channel, physical common packet channel, synchronization channel, common pilot channel, acquisition indication channel, paging indication channel, etc.
The random access channel (RACH) is used to register mobile terminals after power-on to the radio access network or to perform location update after moving form one location area to another or to initiate a call. But the RACH is also used by mobile terminals to send user data at a relatively low data rate. A RACH transmission from a mobile terminal includes a preamble with signature sequence of 16 symbols. The mobile terminal randomly selects one of plural RACH sub-channels from a group the mobile terminal's access class allows it to use and randomly selects one of a group of available predetermined signature sequences. When a base station detects the RACH preamble and the selected signature sequence from the mobile terminal, the base station transmits an acknowledgement of the signature sequence over an acquisition indication channel (AICH) by sending back the same signature sequence as it received from the mobile terminal. Up to 16 signatures may be acknowledged on the AICH at the same time. To detect the AICH acknowledgement, the mobile terminal must of course “hear” the AICH and then obtain a phase reference from the common pilot channel of that base station. Because the AICH also needs to be heard by all mobile terminals, the AICH messages must be transmitted at high power. In case the mobile terminal does not receive an AICH, the mobile terminal increases the preamble transmission power by a step value given by the base station and retransmits the preamble on the RACH in the next available RACH access slot. A similar AICH acknowledgement procedure is performed between the base station and mobile terminals and the common packet channel (CPCH).
The AICH indication messages are transmitted at high power without power control. This can be a problem particularly when there is a heavy load on the random access channel, e.g., a large number of mobile users are leaving an athletic stadium and are making calls at the same time from the same general location. In that kind of situation, there may be a large number of acquisition indications transmitted by the base station during the same access time slot. For a given set of symbol/bit values in the signature sequences, there may be a number of symbol/bit values, which when “constructively” combined and transmitted, require a power output at the base station power amplifier that exceeds a preset maximum power. If that power maximum is exceeded because of the “constructive” power levels generated when multiple signature sequences are transmitted at or close to the same time, negative consequences can occur including damage to the base station power amplifier, distortion of the transmitted signature sequence, and generation of excessive interference in the base station coverage area.
One way to avoid these problems is to only permit the base station to send a maximum number of AICH messages at the same time. This approach may be satisfactory when the load on the RACH/AICH is low to moderate. But at higher loads, this approach does not provide the performance that may be expected or even necessary for certain applications and/or services supported by existing and future mobile communications networks for the base station to send a large number of AICH signals during the same access time slot. One example source of higher loads on the RACH/AICH channel is the Mobile Broadcast Service (MBS).